1. Field of the Invention
This invention generally concerns protection from laser radiation for optical equipment and human eyes and anything else requiring such protection.
2. Description of Related Art
Laser light is a high intensity monochromatic radiation having extremely high coherence. An increasingly large number of applications based on lasers are currently available. Most applications in the consumer sector incorporate low intensity lasers such as compact discs, DVDs and other optical devices, while high intensity lasers largely remain in the research, medical, defense, industrial, nuclear and astronomy sectors. Low energy lasers are very commonly used in law enforcement and warfare for target illumination.
Development of low as well as high power laser systems has reached the maturity where such systems are economically and technically feasible. The main requirement for protection against laser damage of optical receptors, especially in the case of high sensitivity devices, is high transparency under low intensity and ambient conditions combined with opacity under high intensity radiation.
Typically, there are two ways to protect optically sensitive materials against damage caused by overexposure to high intensity light; i.e., active and passive. The active approach is based on “smart structures” comprising electronic circuitry that reacts when a harmful intensity of light is detected and activates mechanical barriers between the light source and the optically sensitive material. The passive approach relies on the inherent property of the material to form a light barrier and prevent transmission of light when its intensity surpasses a threshold value.
Optical power limiters (OPLs) are materials and devices designed to allow normal transmission of light at low intensities and limited transmission of light of higher intensities. A barrier is formed as a direct response of such a material to excessive intensity of light. There are various important considerations that go into the design of an OPL device. The speed at which light travels dictates that an OPL device must be able to react almost instantaneously to changing light intensity. Fast response time favors a materials-based device over a mechanical one. The material must be able to bear the brunt of prolonged exposure to high intensity light, as well as allow for continuous transparency in regions outside the path of the high intensity light. All these factors favor a device incorporating a solution or film of molecules, which acts as a stand-alone optical limiter. Examples of OPL are found in U.S. Pat. Nos. 7,071,279; 6,620,863; 6,522,447; 5,741,442; and US Published Applications 2003/0142397; 2002/0024752; 2007/0107629; and PCT published applications WO/2007/042913 and WO/2005/096726. References that may be of assistance are Brennan, et al., Proceedings of the SPIE, 4876.649-658 (2003). Izard et al., Chem. Phys. Lett. 391, 124-128 (2004); Chin et al., J. Mater. Res. 21, 2758-2766 (2006); Mishra et al., Chem. Phys. Lett. 317, 510-514 (2000); O'Flaherty et al., J. Opt. Soc. Am. B, 20, 49-57 (2003); Xu et al., Macromolecules 39, 3540-3545 (2006); Chen, P. et al., Phys. Rev. Lett. 82, 2548-2551 (1999); Kost et al., Optics Letters 334-336 (1993); and Brustin, G. et al., J. Sol-Gel Sci. Technol. 8, 609-613 (1997).
In order to be used for practical applications, an OPL material must fulfill the following requirements:                1. It must have a fast response time;        2. It should operate over a broad wavelength range; and        3. The on-off cycle must be extremely fast and ideally it should follow the speed of the cycle of the laser pulse it is responding to.        
Optical power limiting devices rely on one or more nonlinear optical (NLO) mechanisms, which include:                1. Excited State Absorption (ESA) or Reverse Saturable Absorption (RSA);        2. Two-Photon Absorption (TPA);        3. Multi-Photon Absorption (MPA);        4. Nonlinear Refraction;        5. Induced Scattering;        6. Photorefraction;        7. Beam Diffusion; and        8. Nonlinear Scattering.        
Nonlinear processes such as excited state or reverse saturable absorption, two-photon and multi-photon absorption, nonlinear refraction, beam diffusion and nonlinear scattering for various materials have been extensively studied for OPL applications. To date, however, there is not yet a single OPL material available which, taken individually, can provide ideal and smooth attenuation of an output beam. Therefore, the design and development of radically novel types of materials for OPL is required. In this regard, some attempts were made with combinations of nonlinear optical materials in cascading geometries, such as multi-plate or tandem cells and use of two intermediate focal planes of a sighting system [see Miles, P. A., Appl. Opt. 31, 6965 (1994); and Van Stryland, E. W. et al., Nonlinear Opt. 27, 181 (2001)].
The human eye is a very sensitive optical sensor with a very low damage threshold for the retina (˜1 μJ). This imposes stringent demands on materials for laser protection. Existing nonlinear optical materials can respond to such low energies only when the light is tightly focused—this is achieved most easily in an optical system which provides focal planes at which the nonlinear material can be positioned. Protection applications demand materials with the following characteristics:                1. High linear transmission across the response band of the sensor;        2. Sensitive nonlinear response to a wide variety of pulse length;        3. Resistance to permanent optical damage; and        4. Stability in the working environment.        
In order to devise such a system various methods have been tried.
Organic materials, such as pthalocyanine derivatives, fullerene, etc. have been found suitable for exhibiting relatively good OPL effects with fast response time. However, these materials have inherent properties that are not appropriate for practical use. For example, they are not stable at high temperatures caused by laser irradiation and will decompose and lose their OPL effect. Moreover, production of designer organic materials for OPL is usually complicated and they can be produced in small quantities only.
Photochromic materials, which reversibly change color in response to light, have also been investigated as OPL materials. The problem with these materials is their slow response. Also, they change their OPL character over time since they keep on responding to weak light (such as ambient light).
Prior art optical limiters use a liquid limiter where chromophores are dissolved in suitable solvents (generally organic solvents). The problems with such limiters are manifold. First, the chromophore concentration cannot be kept constant over time which means the transmittance changes. The chromophores tend to aggregate over time and as a result lose OPL effectiveness. When laser irradiation occurs, the solution starts to move around which makes it difficult to control the refractive index of the medium. Also, it is cumbersome to handle and use liquid limiters in practical devices.
A wide variety of materials, potentially usable as optical limiters, has been tested, including:                Transition metal cluster compounds, such as iron carbonyl cluster compounds, which were blended into a polymer to provide an optical limiting device (see U.S. Pat. No. 5,283,697);        Fullerenes, (see U.S. Pat. Nos. 5,391,329 and 5,741,442); and        Metalloporphyrin and metallophthalocyanine complexes, e.g., lead tetrakis(4-cumylphenoxy) phthalocyanine, which have the strongest RSA effects to date [see J. S. Shirk et al., Appl. Phys. Lett. (63)14, 1880-1882 (1993); and U.S. Pat. No. 5,805,326].        
Some of these complexes were dissolved in poly(methylmethacrylate) (PMMA) and in polystyrene to provide “optical limiter structures”. While these structures can be used as high intensity light attenuators, they are also subject to damage from high intensity light.
Thus, there remains a need for optical limiting devices with superior properties. Therefore, an aspect of the present invention is to provide a method for preparing optical limiting devices that exhibit a strong RSA effect and that are less subject to damage from high intensity light than known devices.